1. Field of the Invention
This invention relates to a contact lens arrangement for optically examining the interior, in particular the retina, of an eye and for treating it by light irradiation.
The eye, in particular the diaphragm, the crystalline and the retina, may be affected by several diseases. To diagnose the affection, the interior of the eye must be optically inspected in an accurate way.
2. Prior Art
Several methods of examination are known. The best images to be had are obtained by using a contact lens which, when applied to the eye, causes minimum distortion of the light beam received from the point under observation, or working point, inside the eye, e.g. a Koeppe or a Goldmann lens, described in particular in a publication entitled "Gonioscopie und Goniofotographie", authored by Winfried Muller and Hans-Peter Brandt, and published by Ferdinand Enke in Stuttgart in 1979.
A Koeppe contact lens has a rounded entry surface and a substantially spherical exit surface adapted to be applied to the cornea. A Goldmann contact lens is more particularly designed for examining the retina and has a plane entry surface and a spherical exit surface. It may also have one or several reflecting surfaces acting as mirrors, enabling indirect examination of the interior of the eye.
Ophthalmologists particularly appreciate these kinds of contact lens for examinations, especially the second, as they are easy to handle and comfortable in use. However, the quality of the image they produce gradually deteriorates as the distance of the point under observation from the the optical axis of the eye increases.
While such a contact lens is useful for observing the interior of an eye, it is essential for treating a retina by irradiation. But in this latter case, because the operating conditions are different from those for observation, the lens must meet stricter requirements.
Treating the interior of an eye by irradiation is achieved by focusing on to the affected area, or onto a part of that area, an intense beam of coherent light. This beam can be produced for example by an Nd-YAG or Argon laser.
For the treatment to be effective and harmless, the beam reaching the working point must be accurately focused and strongly convergent, i.e. the angle made by the light rays at their innermost end should be wide. Good focusing enables the working point to be accurately pin-pointed and considerable energy to be concentrated thereon. Strong convergence enables the working point to be accurately pin-pointed in depth. As a result also, the energy density of the beam decreases rapidly away from the working point, thereby reducing the risk of damaging healthy parts of the eye. The focusing of the light beam would also be better if the spherical aberrations and the astigmatisms of the optical system through which the beam has to travel, i.e. the contact lens and the eye, are slight. However, these flaws usually increase with the diameter or the convergence of the incident beam. These two conditions are thus contradictory and a compromise must be found for each particular use. A Goldmann lens does not solve satisfactorily the problem associated with the wideness of the angle of the light beam, and even less so that associated with astigmatism. Thus, while such a lens will always be suitable for retina examination, retina treatment by irradiation with such a lens will only be satisfactory at points close to the axis of the eye. For other points of the retina the difficulties increase the further removed they are from the axis.
Various improved, more effective, versions have been proposed for the Goldmann lens, e.g. the so-called Abraham lens arrangement consisting of a mirror-less Goldmann lens having a plano-convex lens of mineral glass fitted off-centre to its entry surface. This modification enables the angle of the beam to be widened but tends also to enhance the aberrations. Another example is the so-called Roussel lens arrangement, a more recent development described in the specification of our European patent application 82810044.6. In this lens arrangement the light receiving surface constitutes a wave surface for the beam. As a result, the aberrations are reduced but the width of the angle of the beam is unaffected.
All atempts made to improve the Goldmann lens were aimed at the contact lens itself, without considering the optical properties of the eye it was designed to examine or irradiate. Thus even a flawless contact lens would not enable a light beam to be perfectly focused on a point of the retina remote from the axis of the eye. To reach this point, the beam must travel through the different parts of the eye, in particular the crystalline which has an astigmatism that worsens when viewed with a high angle of incidence. This causes the beam to deteriorate. No contact lens, Goldmann or other, takes this phenomenon into consideration. This is a serious flaw in known lenses designed to treat the retina at points situated away from the axis of the eye.